Mixed bed systems and precoat filter systems have been used in many industrial applications for the purification of aqueous solutions. One application of such systems is in the purification of water for condensate recirculation systems used to drive steam turbines. It is essential that the water be extremely pure to avoid any adverse effects on the surfaces of blades, boilers and pipes of the high pressure steam system.
It is also desirable to reduce the oxygen concentration of water used in condensate recirculation systems and the like. Dissolved oxygen is often removed from feedwater and condensate to reduce the corrosion of metal surfaces in the boiler and condensate lines. Dissolved oxygen can enter the system with the feedwater or, for example, as a result of a pump seal leak or a condenser leak.
Degasifiers and deaerators can be used to remove oxygen from the feedwater (which is preferably deionized) and the condensate. Oxygen can also be removed by adding chemical scavengers including hydrazine, a catalyzed hydrazine or a hydrazine derivative to the feedwater. Sodium sulfite is also an effective oxygen scavenger. However, unlike hydrazine, sodium sulfite contributes to the build up of the total dissolved solids (TDS) in the water. Thus, the use of hydrazine or a hydrazine-like compound is a preferred means of removing oxygen from an aqueous solution.
Hydrazine is a strong reducing agent that reacts with dissolved oxygen according to the following reaction: EQU N.sub.2 H.sub.4 +O.sub.2 .fwdarw.2H.sub.2 O+N.sub.2.
The formation of hydrogen peroxide may be an intermediate step. The reaction is stoichiometric with unreacted hydrazine breaking down at high temperatures to form ammonia and nitrogen.
The use of hydrazine as a chemical oxygen scavenger has several advantages. In particular, hydrazine and oxygen form water and nitrogen which are relatively inert. At elevated temperatures, hydrazine decomposes to form ammonia, which increases the pH in the boiler water and the condensate. This is advantageous because the reaction of hydrazine and oxygen proceeds more rapidly in solutions having relatively basic pH values. In addition, unlike sodium sulfite, the use of hydrazine does not increase the total dissolved solids (TDS) in the solution.
Power plants often add hydrazine to feedwater or condensate to maintain oxygen concentrations at about 20 parts per billion (ppb) with a residual hydrazine concentration of about 50 ppb. The lower the hydrazine/ammonia residual, the higher the oxygen content of the solution.
Hydrazine also reduces the concentration of metal oxides, such as red iron oxide, at the metal surface by the formation of magnetite.
The reaction between hydrazine and oxygen is rather slow at ambient temperatures (about 20 degrees Centigrade (C.)) and at neutral pH values. The reaction rate, however, is influenced significantly by both temperature and hydroxyl ion concentration. An optimum pH for the reaction is about 10. Various organic and organometallic additives are known to accelerate the reaction. Robinson, J.S., Corrosion Inhibitors, Recent Developments, Chemical Technology Review No. 132, Noyes Data Corporation, New Jersey (1979).
Activated carbon is known to catalyze the reaction of hydrazine and oxygen. Ellis et al., The Reaction Between Hydrazine and Oxygen, Presented at the International Conference on Hydrazine and Water Treatment, Bournemouth, England (May 1957) and Houghton et al., The Use of Active Carbon with Hydrazine in the Treatment of Boiler Feed Water, Presented at the International Conference on Hydrazine and Water Treatment, Bournemouth England (May 1957). Granular activated carbon, used in a packed bed, is one means of reducing the reaction time required to reduce oxygen to low levels, as compared to the uncatalyzed reaction.
In such prior art processes, hydrazine is added to a liquid containing dissolved oxygen and the liquid is passed through one or more beds of granular activated carbon to catalyze the reaction between the dissolved oxygen and hydrazine. A more efficient process for the removal of dissolved oxygen would include the use of a finely divided or powdered, rather than granular, activated carbon. However, finely divided materials tend to form densely packed beds.
Thus, a need exists for a process whereby the dissolved oxygen concentration of a hydrazine-containing liquid can be reduced efficiently to low (parts per billion) levels by passage of the liquid through activated carbon in a finely divided state.